It seems like IE showed variations of the website that existed at different points in time. How it does this, I do not know. But I think each of these variations has existed at some point while I was creating the sidebar.

Obviously I repeatedly deleted my cache. Everything worked fine in Firefox and Chrome the entire time.

If someone can explain this, I'm very curious.

Thank you for your advice though esmi. I will look into this and reduce the validation errors on our website to a minimum.